1. Field of the Invention:
The present invention relates to an automatic jet soldering apparatus that can be used for soldering electronic components to a printed circuit board.
2. Description of the Prior Art:
There has been a jet soldering apparatus by which resistors, capacitors, jumper wires, etc. are soldered to a printed circuit board.
The following description deals with the construction and functions of such a conventional soldering apparatus with reference to FIG. 1, wherein a solder bath 1 contains molten solder 2 therein. A nozzle 3 is disposed within the solder bath 1. The nozzle 3 comprises a nozzle body 3a having an open top and an open bottom, and a punched panel 3b having numerous holes through which the molten solder 2 passes and fixed in the lower part of the nozzle body 3a, the punched panel 3b serving to determine the wave height H of the molten solder 2. The nozzle 3 further comprises a back rectifying plate 3c fitted onto the rear portion of the top end of the nozzle body 3a, and a front rectifying plate 3d fitted onto the forward portion of the top end of the nozzle body 3a. The nozzle 3 is connected by a duct (not shown) to a jet source (not shown) having a propeller, the rotation of the propeller causing the molten solder 2 to flow upward through the nozzle body 3a and then along the back rectifying plate 3c and the front rectifying plate 3d to return to the solder bath 1. A printed circuit board 4 is conveyed in the arrow direction (rearward) by means of a belt conveyer or other conveying means, and above the nozzle 3 on the way, the solder side of the printed circuit board 4 is brought in contact with the molten solder 2 for the soldering of resistors and other components. The printed circuit board is conveyed at the same speed as the flow velocity of the molten solder 2 (the solder flow velocity along the back rectifying plate 3c), that is, the conveying speed of the printed circuit board 4 relative to the solder flow velocity becomes zero to create a condition equivalent to static soldering, thus providing highly reliable automatic soldering.
The above construction, however, has the problem that solder residues 5 build up with use on the underside of the punched panel 3b to eventually block the holes provided in the punched panel 3b, which causes turbulence in the wave height H of the solder jet. When such turbulence is caused in the wave height H of the solder jet, the solder cannot be securely applied to the entire surface of the printed circuit board, thus affecting product quality as a result.
To eliminate the above-mentioned problem, it is currently practiced to remove the nozzle 3 from the solder bath 1 for cleaning solder residues from the punched panel 3b. However, since the nozzle 3 is removed from the solder bath 1 having a high temperature of about 250.degree. C., it is not possible to clean the nozzle 3 immediately after removal, and also, cleaning work itself consumes time and labor, which are disadvantages with the construction of such a conventional soldering apparatus.
On the other hand, in the relationship between the flow velocity of the molten solder 2 and the conveying speed of the printed circuit board 4, if the flow velocity of the molten solder 2 is, for example, faster than the conveying speed of the printed circuit board 4, solder will be torn off because of the strong pull of the molten solder 2 on a terminal 4a at the point where the terminal 4a leaves the molten solder 2. This means an increased amount of deposition of the molten solder 2 on the terminal 4a, which brings a problem of the creation of a solder bridge to adjacent terminals. Particularly in printed circuit boards for which higher component density is needed, the spacing between adjacent terminals is becoming increasingly smaller, and a preventive measure against solder bridging is much in need. As an example of such a measure, it is possible to control the solder flow along the back and front rectifying plates 3c and 3d by regulating the rotating speed of the jet source, or to control the conveying speed of the printed circuit board 4. However, when the rotating speed of the jet source is reduced, there arises a problem that the swelling (wave height H) of the molten solder 2 from the nozzle 3 is insufficient, preventing the full contact of the molten solder 2 with the solder side of the printed circuit board 4. Conversely, when the rotating speed of the jet source is increased, there arises a problem that the wave height H of the molten solder 2 becomes excessively high, causing the molten solder 2 to reach the component side of the printed circuit board 4 with a resultant trouble in practical use. It is therefore desired to keep a consistent wave height H of the molten solder 2. On the other hand, it is also desired to keep a consistent conveying speed of the printed circuit board 4 from the viewpoint of production efficiency. Therefore, it has been extremely difficult with the construction of the conventional soldering apparatus to achieve a zero relative speed between the printed circuit board 4 and the solder flow velocity.